Using a single-photon-emission computerized tomographic scanner (SPECT) the applicants will study regional cerebral blood flow (rCBF) in human volunteers under normal and disease circumstances. SPECT derives and displays rCBF in three transverse tomographic slices centered 4 cm apart following the one-minute inhalation of l33Xe and a three-minute washout period. rCBF values are calculated by a modified Kety-Schmidt method and are expressed in ml/100 g/min. Normal rCBF patterns will be evaluated in a group of volunteers, and the same volunteers will undergo "lateralization" tests such as motor and cognitive activities, as well as presumed "right-sided" activities. Several groups of patient volunteers will be studied, including (1) patients who have had strokes, or who are known to have cerebrovascular disease; (2) patients with known seizure disorder; (3) patients with unipolar and bipolar depression, and schizophrenia. Patients in group (1) ought to have measurable flow disturbances, while patients in groups (2) and (3) may have rCBF abnormalities, according to earlier work done by other groups. Thus far distinctive patterns have been obtained in patient-volunteers in groups (1), (2) and (3). Applicants will attempt to evalute SPECT determination of rCBF as a possible clinical technique for wide deployment. Its diagnostic utility and cost/benefit will be appraised, and comparison will be made with information gained with positron-emission romographic systems. Perhaps for some purposes SPECT determination of rCBF might be a relatively low-cost substitute for positron technology.